1. Field of the Invention
The present invention relates to a process for preparing primarily syndiotactic crystalline vinyl aromatic polymers. More particularly, the present invention relates to a process for preparing crystalline styrene polymers in which the chains are at least primarily in the syndiotactic configuration.
As is known, styrene polymers, and particularly polystyrene, are thermoplastic materials obtainable in high molecular weights. Their thermal resistance, susceptibility to thermal molding and excellent insulating properties render them particularly suitable for the production of extruded articles, molded articles and films to be used chiefly in the fields of sound and heat insulation for packings and as dielectric materials.
2. Description of the Prior Art
Polystyrene can be produced by free radical polymerization, anionic polymerization or cationic polymerization.
The polymerization of styrene conducted by the free radical mechanism, either thermally or in the presence of initiators, can be effected by the techniques of bulk polymerization, emulsion polymerization, suspension polymerization or bulk suspension polymerization. On x-ray analysis, the resulting polymer exhibits a diffraction pattern typical of an amorphous structure, and on .sup.1 H-NMR analysis it exhibits a spectrum with broad bands without the resolution of peaks in the area of the chemical shifts, typical of the methine and methylene protons, in accordance with a random sequence of monomeric units with different steric configuration (atactic structure).
The field of use of this amorphous polymer is limited by the low glass transition temperature (Tg) which is about 100.degree. C. Above this temperature, in fact, the mechanical properties of the polymer drastically decay.
Amorphous polymers are also obtained when the polymerization of styrene is conducted by cationic and anionic catalysis, even though in some cases the carbon atoms in the main chain form a prevalent syndiotactic configuration.
It is also known that crystalline, primarily isotactic polystyrene can be produced by the stereospecific polymerization of styrene. Styrene polymerization, in such case, is carried out in the presence of Ziegler-Natta-type catalysts based on halogenated compounds of transition metals, either as such or supported on magnesium halides, in combination with aluminum alkyls, optionally in the presence of electron donors. Detailed descriptions of this type of polymerization are given in U.S. Pat. Nos. 3,161,624 and 2,882,263, and in British Pat. Nos. 826,021 and 844,944. The polystyrene prepared by such stereospecific catalysis was first obtained by Natta et al. (J. Am. Chem. Soc. 1955, 77, 1700) and shows, in the x-ray pattern, diffraction peaks attributable to primarily isotactic-type structure. On .sup.1 H-NMR analysis, the multiplet ascribed to the methylene group is indicative of non-equilvalence of the two protons, which is also indicative of an isotactic structure (Heatley F., Bovey F.A. in Macromolecules (1968), 1, 301). This crystalline, primarily isotactic polymer has not found any industrial applications since its crystallization rate is very low.
Known too (N. Ishirara et al., Macromolecules (1986), 19, 2464-2465) is a crystalline polystyrene having a low weight average molecular weight (about 82,000) and primarily a syndiotactic structure, attributed on the basis of its x-ray diffraction, infrared spectroscopic analysis, .sup.1 H-NMR and .sup.13 C-NMR analyses. This polymer has a high melting point, about 270.degree. C., and a high crystallization rate.
The thermal and structural characteristics of the aforesaid primarily syndiotactic polymers are such that their mechanical properties are unaltered even at temperatures higher than the glass transition temperature (Tg).
It is well known, however, that many mechanical properties of polystyrene, such as tensile strength, elongation at break, tenacity and stress-cracking resistance, improve remarkably as the molecular weight increases, while they drop to values which are unacceptable for practical industrial utilization when the weight average molecular weight is below 100,000 (Encyclopedia of Polymer Science and Technology, Vol. 13). For this reason, commercial styrene polymers have weight average molecular weights ranging from 200,000 to 300,000.